1 2 3 4 A - SIPROTEC

SIPROTEC, 7UT6x, Manual
C53000-G1176-C230-2, Release date 06.2012
Functions
2.2 Differential Protection
Figure 2-31 shows an example of an earth fault on the delta side outside the protected zone if an earthed
starpoint former (zigzag winding) is installed within the protected zone. Here, a zero sequence current occurs
on the right side but not on the left, as above. If the starpoint former were outside the protected zone (i.e. CTs
between power transformer and starpoint former), the zero sequence current would not pass through the
measuring point (CTs) and would not have any harmful effect.
Figure 2-31 Example of an earth fault outside the protected transformer with a neutral earthing reactor
within the protected zone
The disadvantage of elimination of the zero sequence current is that the protection becomes less sensitive
(factor 2 / 3 because the zero sequence current amounts to 1 / 3 in case of an earth fault in the protected area.
Therefore, elimination is suppressed in case the starpoint is not earthed (see figure 2-28), or the starpoint
current can be included (figure 2-29). If, for example, a surge voltage arrester is connected to the starpoint, one
should do without the advantage of that option in order to avoid recognition of a breakdown of the surge voltage
arrester as an internal fault. For this purpose, the starpoint of the respective side must be set to Earthed
(addresses 313 STARPNT SIDE 1, 323 STARPNT SIDE 2, 333 STARPNT SIDE 3, 343 STARPNT SIDE 4,
353 STARPNT SIDE 5).
Use on Auto-Transformers
In order to achieve comparable currents for the differential protection, all currents are referred to the winding
(= side) with the highest apparent power rating. This apparent power is named the rated power of the protected
object bezeichnet. If the highest apparent power rating occurs repeatedly, that side which has the higher rated
current will be selected as the reference side, unlike the other protected objects.
Auto-connected windings in auto-transformers can only be connected Y(N)y0 (figure 2-32). If the starpoint is
earthed, all auto-connected windings connected to the system parts (higher and lower voltage system) are
affected. The zero sequence system of both system parts is coupled because of the common starpoint.
Figure 2-32 Auto-transformer with earthed starpoint
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